1. Field of the Invention
The present invention relates to an apparatus and method for controlling oxygen sensor heating to restrict sensor deterioration caused by excessive heating.
2. Description of Related Art
Many modern air-fuel ratio control systems for engines use a current limiting type oxygen sensor (oxygen concentration detector) which produces current proportional to oxygen concentration in exhaust gas.
This type of oxygen sensor is incapable of producing current proportional to the oxygen concentration until activated at predetermined temperature (activation temperature). Therefore, to maintain the activation of the oxygen sensor, power supply to a heater for heating the oxygen sensor is controlled in response to temperature of the sensing element in the oxygen sensor (element temperature feedback control).
The feedback control for the oxygen sensor temperature is based on the sensing element impedance which varies with temperature. The target impedance is set to the impedance at which the activation temperature is attained.
Internal impedance of the sensing element increases as the oxygen sensor deteriorates. Therefore, even when the temperature of the oxygen sensor is above the activation temperature, the supply power to the heater (heater supply power) will be increased to attain the target impedance because the impedance of the sensing element increases by deterioration. The temperature of the oxygen sensor will accordingly rise. Further, when the oxygen sensor operating characteristics change due to sensor deterioration or the sensor environment (e.g., engine exhaust gas temperature) changes, the heater supply power will become unstable and cause excessive heating of the sensing element. Thus, this temperature rise will adversely promote deterioration of the oxygen sensor.